1. Field of the Invention
The present invention relates generally to digital data detectors and, more particularly, to a digital data detector for detecting a data detection clock signal and digital data from a digital receive signal in a digital data transmission system.
2. Description of the Background Art
Conventionally, various methods for detecting digital data from a digital receive signal have been proposed. One example will be described in the following. That is, a digital receive signal is sampled at a frequency of m times (m&gt;1) a channel bit rate. Then, from sampling data S.sub.i+1 (hereinafter referred to as present data) of a sampling point R.sub.i+1 at which a phase is calculated at present (hereinafter referred to as the present sampling point) and sampling data S.sub.i (hereinafter referred to as preceding data) of a preceding sampling point R.sub.i of the sampling point R.sub.i+1 (hereinafter referred to as the preceding sampling point), an interval P between a point at which the receive signal crosses a zero level (hereinafter referred to as a zero cross point) and the presence sampling point R.sub.i+1 is obtained by the following expression (1). EQU P=.vertline.S.sub.i+1 .vertline./(.vertline.S.sub.i+1 .vertline.+.vertline.S.sub.i .vertline.).times.(N/2) (1)
where N is the value of a phase representing a channel bit interval. A predicted value P.sub.i+1 ' of the phase of the present sampling point R.sub.i+1 (hereinafter referred to as the present phase predicted value) which is obtained from the value P and from a phase P.sub.i (hereinafter referred to as the preceding phase) of the preceding sampling point R.sub.i and a phase P.sub.i-2 of a sampling point R.sub.i-2 which is two sampling points before the sampling point R.sub.i+2, is calculated by the following expression (2). EQU P.sub.i+1 '={(P.sub.i -P.sub.i-2).times.L+P.sub.i +(N/2)} mod N (L is a constant: 0&lt;L&lt;1) (2)
By use of this phase predicted value P.sub.i+1 ', a phase P.sub.i+1 (hereinafter referred to as the present phase) of the present sampling point R.sub.i+1 is obtained. When there is a zero cross point between the present sampling point R.sub.i+1 and the preceding sampling point R.sub.i, the above P.sub.i+1 is obtained by the following expression (3). EQU P.sub.i+1 ={(P-P.sub.i+1 ').times.K+P.sub.i+1 '} mod N (K is a constant: 0&lt;K&lt;1) (3)
On the other hand, when there is no zero cross point between the present sampling point R.sub.i+1 and the preceding sampling point R.sub.i, the above P.sub.i+1 is obtained by the following expression (4). EQU P.sub.i+1 =P.sub.i+1 ' (4)
A data detection clock signal and digital data are detected on the basis of the result of comparison between P, P.sub.i and P.sub.i+1 obtained by the above expressions (1) to (4) and the following clock signal extracting conditions and data determining conditions. This is disclosed in Japanese Patent Laying-Open No. 01-025357. EQU P.sub.i &gt;P.sub.i+1, P.sub.i .gtoreq.N/2, and P.sub.i+1 .gtoreq.N/2: There is a clock signal. (5) EQU P.sub.i &gt;P.sub.i+1, P.sub.i &lt;N/2, and P.sub.i+1 &lt;N/2: There is a clock signal. (6) EQU P.sub.i &lt;P.sub.i+1, P.sub.i &lt;N/2, and P.sub.i+1 .gtoreq.N/2: There is a clock signal. (7) EQU When P&gt;P.sub.i+1 -N/2, digital data is opposite to an MSB (Most Significant Bit) of S.sub.i ; otherwise, the digital data is the MSB of S.sub.i ( 8)
FIG. 3 is a diagram for use in explaining problems which occur when data is transmitted by using a conventional method. Now assume that when data shown in FIG. 3 (a) is transmitted via a certain transmission path, distortion occurs due to characteristics of the transmission path and that a receive signal shown in FIG. 3 (b) is received on a receiver side. (With respect to transmission data in this case, a minimum inversion spacing T min is equal to a detection window width Tw.) In a case the result obtained by sampling this receive signal at a double frequency of the channel bit rate is input to a data detector by a conventional method. Here, a channel bit interval is equally divided into 64 portions to represent a phase (i.e., N=64) and both of the foregoing coefficients K and L are 0.25.
First, a phase interval P between the present sampling point R.sub.i+1 and the zero cross point is obtained on the basis of the foregoing expression (1). The expression (1) is effective only when a zero cross point exists between the present sampling point R.sub.i+1 and the preceding sampling point R.sub.i. Referring to FIG. 3, sampling points R.sub.1 and R.sub.8 meet this condition, and the phase interval P at this time is as shown in FIG. 3 (d) below. EQU Sampling point R.sub.1 : P=.vertline.S.sub.1 .vertline./(.vertline.S.sub.1 .vertline.+.vertline.S.sub.0 .vertline.).times.(N/2)=30/(30+23).times.32=18 EQU Sampling point R.sub.8 : P=.vertline.S.sub.8 .vertline./(.vertline.S.sub.8 .vertline.+.vertline.S.sub.7 .vertline.).times.(N/2)=5/ (5+47).times.32=3
Focusing on the sampling point R.sub.1, a predicted value P.sub.1 ' of the present phase is as follows in accordance with the foregoing expression (2). EQU P.sub.i '=(P.sub.0 -P.sub.-2).times.L+{(P.sub.0 +N/2) mod N}=(23-23).times.0.25+(23+32) mod 64)=55
(X mod Y means a remainder of X/Y.)
By use of this result and the foregoing computation result of P, the present phase P.sub.1 is obtained by the expression (3). EQU P.sub.i ={(P-P.sub.1 ').times.K+P.sub.1 '} mod N={(18-55).times.0.25+55} mod 64=61
(The value of the phase is 0 to N-1; and when the phase value exceeds N/2, computation is carried out by using a value obtained by subtracting N from the resultant value.) In the case with a sampling point R.sub.2, since there is no zero cross point between the present sampling point R.sub.2 and the preceding sampling point R.sub.1, a predicted value P.sub.2 ' of the present phase is the present phase P.sub.2. EQU P.sub.2 =P.sub.2 '=(P.sub.1 -P.sub.i-1).times.L+(P.sub.1 +N/2)=(61-55).times.0.25+(61+32)=30
Similarly, if P, P.sub.i+1 ' and P.sub.i+1 are obtained with respect to sampling points R.sub.3 -R.sub.9, results are as shown in FIG. 3 (d)-(f). If these results are compared with the above-described clock signal extracting conditions and data determining conditions (the expressions (5)-(8)), a data detection clock signal and digital data are as shown in FIG. 3 (g) and (h). These results do not match transmission data and are hence erroneous data. As can be seen from FIG. 3 (b), the receive signal has a frequency variation, and the above data error is due to this frequency variation.
As described above, in the conventional method, when there is a frequency variation in digital receive data as in a system for recording and reproducing digital data on a magnetic tape, for example, there is the problem that a data error is liable to occur, resulting in a degradation in reliability of the system. This is because particularly when a zero cross point has not been detected over a long period of time upon obtaining a predicted value of a phase, phase information in the past has not been sufficiently reflected on the predicted value of the phase. In FIG. 3 (b), for example, since the phase is corrected with sampling data at the sampling point R.sub.1, the phase interval between the sampling points R.sub.1 and R.sub.0 is 38, which is broader than the case where there is no frequency variation (N/2=32 when there is no frequency variation). However, the phase interval between sampling points R3 and R4 is 32, and hence, the result which is corrected at the sampling point R.sub.1 is not reflected.